The invention relates to an electric lamp comprising a light-transmitting lamp vessel in which a light source is arranged,
wherein at least a portion of the lamp vessel is provided with an interference film for allowing passage of visible-light radiation and reflecting infrared radiation,
wherein the interference film comprises layers which are arranged such that a first layer of a material having a first refractive index alternates with a second layer of a material having a second refractive index,
wherein the second refractive index is comparatively high as compared to the first refractive index.
The invention further relates to an interference film for use in an electric lamp.
Said interference films reflect and/or allow passage of radiation originating from different parts of the electromagnetic spectrum, for example ultraviolet, visible and/or infrared light. Such interference films are customarily provided as a coating on (the lamp vessel of) electric lamps and/or on reflectors. In this manner, the efficiency of an electric lamp is increased by applying a coating which reflects the infrared light, but which allows passage of the light radiation which is visible to the naked eye. The geometry of the electric lamp and/or of the reflector is such that the reflected radiation is reflected back to the light source which comprises, for example, a filament or a discharge. The reflected (heat) radiation is customarily used to assist in maintaining the operating temperature of the light source, thereby improving the energy balance of the electric lamp.
An electric lamp of the type mentioned in the opening paragraph is known from U.S. Pat. No. 5,138,219. In the known electric lamp, the material of the first layer consists of silicon oxide, and the material of the second layer consists of tantalum oxide having a refractive index which is high as compared to that of silicon oxide.
It is a drawback of the electric lamp that the degree to which the radiation reflected by the interference film contributes to the energy balance of the electric lamp is comparatively small, so that the efficiency increase also is comparatively small.
It is an object of the invention to provide an electric lamp of the type described in the opening paragraph, which electric lamp has an improved efficiency.
In accordance with the invention, the lamp of the type described in the opening paragraph is characterized in that, in the wavelength range from 400 to 760 nm, the interference film has a transmittance, on average, of at least 90%, and in the wavelength range from 800 to 2200 nm, the interference film has a reflectance, on average, of at least 75%.
An interference film with a high transmittance in the visible-light range (400-760 nm) and, simultaneously, a high reflectance in a comparatively wide part of the infrared range (800-2200 nm) of the electromagnetic spectrum leads to an increase of the quantity of (heat) radiation reflected by the interference film, which results in an improvement of the efficiency of the electric lamp. At a corresponding transmittance in the wavelength range from 400 to 760 nm, the known electric lamp has a reflectance in the infrared range which is lower than that of the lamp in accordance with the invention. In addition, this lower reflectance of the interference film of the known electric lamp is obtained in a wavelength range which is much smaller than that of the invention. The reflectance of the interference film in the known electric lamp is, on average, approximately 70% in a wavelength range from 800 to 1900 nm, while the reflectance of the interference film in the electric lamp in accordance with the invention is, on average, at least 75% in the wavelength range from 800 to 2200 nm.
Preferably, the reflectance in a wavelength range from 800 to 2500 nm is, on average, at least 85%. An interference film with such a high reflectance in a comparatively wide part of the infrared range (800-2500 nm) of the electromagnetic spectrum leads to a substantial increase of the quantity of (heat) radiation reflected by the interference film, which results in a substantial improvement of the efficiency of the electric lamp.
In conventional interference films for allowing passage of visible-light radiation and reflecting infrared radiation, which interference films are based on two materials having mutually different refractive indices, a plurality of conventional multilayer stacks of various design wavelengths are employed. Such conventional stacks are composed of three successive layers, namely a first layer (L) of the material having the first refractive index, a second layer (H) of the material having the second refractive index, and a third layer (L) of the material having the first refractive index. Such a conventional three-layer stack is represented in the following manner, which is known to those skilled in the art:       (                                        L            a                                                H            d                                                L            a                                )    x
where
1xe2x89xa6axe2x89xa62,
1xe2x89xa6dxe2x89xa62,5,
1xe2x89xa6xxe2x89xa620.
A drawback of the application of such a conventional stack resides in that the spectral width of the transmission window in the visible range is comparatively small. An increase of the effectiveness of the interference film in the infrared range of the electromagnetic spectrum is hardly possible and leads to a further narrowing of the spectral width of the transmission window in the visible range and, in addition, to undesirable interference peaks in the visible range.
The spectral width of the transmission window in the visible range can be increased in a manner which is known per se by using interference films based on three materials instead of two, the refractive index of the third material ranging between the refractive index of the first material and the refractive index of the second material. A layer comprising such a third layer material is often referred to as an intermediate layer (M). Although such a three-material approach to the interference film is practicable per se, it is not easy to find three materials which are suitable for said purpose and which, in the course of (industrially) applying the interference film to the lamp vessel of the electric lamp, can also be provided (sufficiently) independently of each other.
In the known lamp""s interference film (U.S. Pat. No. 5,138,219) for allowing passage of visible-light radiation and reflecting infrared radiation, the third, intermediate layer material (M) is simulated by a combination of the two other layer materials. In this case, a three-material, five-layer stack of the structure:   (                              L          a                                      M          b                                      H          d                                      M          b                                      L          a                      )
where
2xe2x89xa6a, bxe2x89xa64,
1xe2x89xa6dxe2x89xa62,5,
is converted to a two-material, seven-layer H-L stack of the structure:   (                              L          a                                      HL                      b            xe2x80x2                                                H          d                                      LH                      b            xe2x80x2                                                L          a                      )
where
2xe2x89xa6axe2x89xa64,
5xe2x89xa6bxe2x80x2xe2x89xa615,
1xe2x89xa6dxe2x89xa62,5.
The drawback of interference films wherein such seven-layer stacks of two materials are employed resides in that the effective infrared reflectance is limited to a wavelength range from approximately 800 to 1900 nm. If it is necessary to also reflect infrared radiation of a longer wavelength (xcex greater than 1900 nm), it is found that such seven-layer H-L stacks, considering the desired design wavelengths, cause undesirable side effects in the visible range, which lead to a decrease of the transmission window in the visible range as well as to the development of interference peaks in the center of the visible range. Such side effects adversely affect the appearance of the electric lamp and the color rendition of the light emitted by the light source. These interference peaks are so-called fifth-order reflectance peaks, which can be suppressed by means of a multilayer stack comprising two intermediate layer materials. Such a stack has the following structure:   (                              L          a                                                  M            1                                b            1                                                            M            2                                b            2                                                H          d                                                  M            2                                b            2                                                            M            1                                b            1                                                L          a                      )
where
2xe2x89xa6axe2x89xa64,
2xe2x89xa6b1,b2xe2x89xa64,
1xe2x89xa6dxe2x89xa62,5.
If finding one suitable intermediate layer material for the above-mentioned five-layer stack is substantially impossible, then this applies most certainly to finding, for this seven-layer stack, two suitable layer materials having a refractive index ranging between that of the layer having a high refractive index and the layer having a low refractive index. In the current invention, each one of these intermediate layer materials M1 and M2 is simulated by a suitable combination of layers of the other two layer materials, i.e. H and L.
A preferred embodiment of the electric lamp is characterized in that the interference film comprises at least a multilayer stack of at least eleven layers which are arranged such that a first layer (L) of the material having the first refractive index alternates with a second layer (H) of the material having the second refractive index, which multilayer stack has the following structure:   (                              L          a                                      HL          b                                      HL          c                                      H          d                                      LH          c                                      LH          b                                      L          a                      )
where
2xe2x89xa6axe2x89xa64,
5xe2x89xa6bxe2x89xa650,
5xe2x89xa6cxe2x89xa650,
1xe2x89xa6dxe2x89xa63,
Using such eleven-layer H-L stacks, the width of the infrared wavelength range that is effectively reflected can be substantially increased to a range from 800 to 2200 nm, preferably from 800 to 2500 nm, without an appreciable reduction of the transmittance in the visible range. Such an increase in width leads to an increase of the quantity of (heat) radiation reflected by the interference film, which results in an improved efficiency of the electric lamp. In addition, the use of such eleven-layer H-L stacks enables the effectiveness of the infrared reflection to be substantially increased throughout the effective wavelength range.
In the case of interference films based on two layer materials (H and L), the increase in reflectance to values above, on average, 80% in the infrared wavelength range without an extension into the far infrared range (xcexxe2x89xa71900 nm) generally leads to a substantial narrowing of the transmission window in the visible range (below 300 nm). The conversion of such filter designs for interference films in order to obtain a xe2x80x9cnormalxe2x80x9d transmission window in the visible range leads to a loss of reflectance. The invention is based on the recognition that, while the desired xe2x80x9cnormalxe2x80x9d transmission window in the visible range (400-760 nm) is preserved, the infrared reflection is increased to values of, on average, 85% or higher, and also the width of the reflection band in the infrared wavelength range is increased so as to range from 800 to 2200 nm, preferably from 800 to 2500 nm.
A preferred embodiment of the electric lamp is characterized in that the interference film comprises at least one further multilayer stack of at least seven layers, which are arranged such that the first layer (L) of the material having the first refractive index alternates with the second layer (H) of the material having the second refractive index, which further stack has the following structure:   (                              L                      a            xe2x80x2                                                HL                      b            xe2x80x2                                                H                      d            xe2x80x2                                                LH                      b            xe2x80x2                                                L                      a            xe2x80x2                                )
where
2xe2x89xa6axe2x80x2xe2x89xa64,
5xe2x89xa6bxe2x80x2xe2x89xa650,
1xe2x89xa6dxe2x80x2xe2x89xa63,
The material of the first layer (L) having the first refractive index preferably mainly comprises silicon oxide.
The material of the second layer (H) having the second refractive index is preferably chosen from the group formed by titanium oxide, niobium oxide, zirconium oxide, hafnium oxide, tantalum oxide. These layers having a high refractive index may alternatively consist of a combination of two layers, for example of TiO2xc3x97ZrO2, TiO2xc3x97HfO2, TiO2xc3x97Nb2O5, TiO2xc3x97Ta2O5 or Ta2O5xc3x972TiO2.
It has been found that an electric lamp provided with an interference film in accordance with the invention based on Nb2O5 as the material having the high refractive index and SiO2 as the material having the low refractive index retains its initial properties throughout the service life of the lamp. In an alternative interference film, a mixture of Nb2O5xc3x97Ta2O5 is used, which is particularly suitable if the operating temperature of the electric lamp is comparatively high. Experiments have shown that the filter design of such an interference film hardly needs adapting.
The light source of the lamp may be an incandescent body, for example in an halogen-containing gas, but it may alternatively be an electrode pair in an ionizable gas, for example an inert gas with metal halides, possibly with, for example, mercury as the buffer gas. The light source may be surrounded by an innermost gastight envelope. It is alternatively possible that an outermost envelope surrounds the lamp vessel.
The interference film can be provided in a customary manner, for example by physical vapor deposition (PVD) or (dc) (reactive) sputtering or by means of dip coating or LP-CVD (low-pressure chemical vapor deposition), PE-CVD (plasma-enhanced CVD).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.